The present invention relates to digital signal processing and, in particular, to a particularly robust watermark mechanism by which identifying data can be encoded into digital signals such as audio or video signals such that the identifying data are not perceptible to a human viewer of the substantive content of the digital signals yet are retrievable and are sufficiently robust to survive other digital signal processing.
Video and audio data have traditionally been recorded and delivered as analog signals. However, digital signals are becoming the transmission medium of choice for video, audio, audiovisual, and multimedia information. Digital audio and video signals are currently delivered widely through digital satellites, digital cable, and computer networks such as local area networks and wide area networks, e.g., the Internet. In addition, digital audio and video signals are currently available in the form of digitally recorded material such as audio compact discs, digital audio tape (DAT), minidisc, and laserdisc and digital video disc (DVD) video media. As used herein, a digitized signal refers to a digital signal whose substantive content is generally analog in nature, i.e., can be represented by an analog signal. For example, digital video and digital audio signals are digitized signals since video images and audio content can be represented by analog signals.
The current tremendous growth of digitally stored and delivered audio and video is that digital copies which have exactly the same quality of the original digitized signal can easily be made and distributed without authorization notwithstanding illegality of such copying. The substantive content of digitized signals can have significant proprietary value which is susceptible to considerable diminution as a result of unauthorized duplication.
It is therefore desirable to include identifying data in digitized signals having valuable content such that duplication of the digitized signals also duplicates the identifying data and the source of such duplication can be identified. The identifying data should not result in humanly perceptible changes to the substantive content of the digitized signal when the substantive content is presented to a human viewer as audio and/or video. Since substantial value is in the substantive content itself and in its quality, any humanly perceptible degradation of the substantive content substantially diminishes the value of the digitized signal. Such imperceptible identifying data included in a digitized signal is generally known as a watermark.
Such watermarks should be robust in that signal processing of a digitized signal which affects the substantive content of the digitized signal to a limited, generally imperceptible degree should not affect the watermark so as to make the watermark unreadable. For example, simple conversion of the digital signal to an analog signal and conversion of the analog signal to a new digital signal should not erode the watermark substantially or, at least, should not render the watermark irretrievable. Conventional watermarks which hide identifying data in unused bits of a digitized signal can be defeated in such a digital-analog-digital conversion. In addition, simple inversion of each digitized amplitude, which results in a different digitized signal of equivalent substantive content when the content is audio, should not render the watermark unreadable. Similarly, addition or removal of a number of samples at the beginning of a digitized signal should not render a watermark unreadable. For example, prefixing a digitized audio signal with a one-tenth-second period of silence should not substantially affect ability to recognize and/or retrieve the watermark. Similarly, addition of an extra scanline or an extra pixel or two at the beginning of each scanline of a graphical image should not render any watermark of the graphical image unrecognizable and/or irretrievable.
Digitized signals are often compressed for various reasons, including delivery through a communications or storage medium of limited bandwidth and archival. Such compression can be lossy in that some of the signal of the substantive content is lost during such compression. In general, the object of such lossy compression is to limit loss of signal to levels which are not perceptible to a human viewer or listener of the substantive content when the compressed signal is subsequently reconstructed and played for the viewer or listener. A watermark should survive such lossy compression as well as other types of lossy signal processing and should remain readable within in the reconstructed digitized signal.
In addition to being robust, the watermark should be relatively difficult to detect without specific knowledge regarding the manner in which the watermark is added to the digitized signal. Consider, for example, an owner of a watermarked digitized signal, e.g., a watermarked digitized music signal on a compact disc. If the owner can detect the watermark, the owner may be able to fashion a filter which can remove the watermark or render the watermark unreadable without introducing any perceptible effects to the substantive content of the digitized signal. Accordingly, the value of the substantive content would be preserved and the owner could make unauthorized copies of the digitized signal in a manner in which the watermark cannot identify the owner as the source of the copies. Accordingly, watermarks should be secure and generally undetectable without special knowledge with respect to the specific encoding of such watermarks.
What is needed is a watermark system in which identifying data can be securely and robustly included in a digitized signal such that the source of such a digitized signal can be determined notwithstanding lossy and non-lossy signal processing of the digitized signal.
In accordance with the present invention, robustness of watermark data is enhanced by precoding and convolving the watermark data. Precoding, using a 1/(1 XOR D) precoder, produces inversion-robust watermark data. 1/(1 XOR D) precoding is known and is sometimes generally referred to as Alternative Mark Inversion (AMI) coding and as bipolar coding. The result of such coding is inversion-robust watermark data. As used herein, xe2x80x9cinversion-robustxe2x80x9d means that decoding the inversion-robust watermark data results in the same watermark data regardless of whether the inversion-robust watermark data has been inverted. In other words, decoding the inversion-robust watermark data by applying an inverse 1/(1 XOR D) precoder produces the original watermark data, and decoding the inversion-robust watermark data after it has been inverted by applying an inverse 1/(1 XOR D) precoder to the inverted inversion-robust watermark data produces the original watermark data. Inversion robustness is particularly important for watermarking audio signals since an inverted audio signal sounds identical to the original audio signal when played back for a human listener.
Convolution of the inversion-robust watermark data results in multiple bits which collectively represent the logical value of a single, corresponding bit of the inversion-robust watermark data. As a result, the convolved watermark data is more robust in that the convolved watermark data is more likely to be recognizable after significant lossy processing. In particular, decoding individual bits of the convolved watermark data produces correlation signals which are frequently not easily interpreted as either a logical one or a logical zero. Using several encoded convolved bits to represent each bit of the inversion-robust watermark data increases the likelihood that each such bit is accurately interpreted.
In addition, the convolution is performed in such a manner that inversion of the encoded convolved watermark data causes decoding to produce inverted inversion-robust watermark data. Specifically, convolution of a particular bit of the inversion-robust watermark data involves a number of convolved bit generators, each of which generates a parity bit based on a number of bits within a few bit positions of the particular bit within the inversion-robust watermark data. To ensure proper interpretation of an inverted watermark signal, each convolved bit generator uses an odd number of bits to determine parity. Accordingly, inversion of the encoded, convolved watermark data changes the parity and therefore decodes to produce inverted inversion-robust watermark data. For example, if an odd number of bits includes an even number of logical one bits, there are also an odd number of logical zero bits. Conversely, if an odd number of bits includes an odd number of logical one bits, there are also an even number of logical zero bits.
Each of the convolved bits is encoded into a basis signal in a manner that ensures that inversion of the watermarked signal inverts the encoded watermark data as well. In particular, watermark data is encoded in a basis signal by division of the basis signal into segments and inverting the basis signal in segments corresponding to watermark data bits with a first logical value and not inverting the basis signal in segment corresponding to watermark data bits with a different logical value. In particular, the basis signal is generated independently of the watermark data.
While 1/(1 XOR D) precoding and convolutional encoding each independently significant improve the robustness and survivability of an encoded watermark in a watermarked signal, combination of both mechanisms results in a particularly robust watermark signal. Accordingly, watermark data can be recognized more reliably within a watermarked signal notwithstanding significant lossy processing of the watermarked signal.